Thick film precision resistor for use in an electrical circuit and method of making same

ABSTRACT

A THICK FILM RESISTOR IS FORMED BY #1 FIRING SELECTED MIXTURES OF POSITIVE AND NEGATIVE THICK FILM RESISTOR INK MATERIALS ON A NON-ELECTRICALLY CONDUCTIVE SUBSTRATE AT A SELECTED HIGH TEMPERATURE AND #2 SUBSEQUENTLY JOINTLY FIRING THE RESISTOR AND CONDUCTOR INK MATERIAL ASSOCIATED WITH THIS RESISTOR, AT A TEMPERATURE THAT IS LOWER THAN THE FIRST MENTIONED TEMPERATURE AND WHICH IS AT A LEVEL THAT WILL NOT ALLOW ANY DETRIMENTAL DIFFISION, TO OCCUR BETWEEN THE CONDUCTOR AND RESISTOR MATERIALS. THIS UNIQUE PROCESS OF FIRING ACHIEVES FOR THE FIRST TIME A SUCCESSFUL METHOD OF FIRING A RESISTOR AND CONDUCTOR JOINTLY WITHOUT ALLOWING ANY ADVERSE CHANGE TO OCCUR IN THE VALUE OF THE TEMPERATURE COEFFICIENT OF RESISTIVITY AND THE RESISTIVITY VALUE OF THE RESISTOR.

Jan.29,1974

Filed March 2,

s. SCHEBALIN 3,783,391 THICK FILM PRECISION RESlS'l'UR FOR USE IN ANELECTRICAL CIRCUIT AND METHOD OF MAKING SAME 5 Sheets-Sheet 1 ZERO TCR ll I 90 70 y 50 30 I I0 I00 80 6O 4O 20 9 7 5 POSITIVE .TCR INK INVENTORSCHEBAUN AGENT SERGEI Jan. 29, 1974 S. SCHEBALIN 3 7 91 THICK FILMCISION RESISTOR FOR USE IN AN ELECTRICAL CI UIT AND METHOD OF MA FiledMarch 2, 1971 KING SAME 5 Sheets-Sheet 2 A we m E H V C W. S U G R E SAGENT.

Jan. 29, 1974 S. SCHEBALI N 3 ?8,8Q1

THICK FILM PRECISION RESISTOR FOR USE IN AN ELECTRICAL Filed March 2 TCRPPM/C CIRCUIT AND METHOD OF MAKING SAME 5 Sheets-Sheet O O (l) I 1 I i IIIIII IIIIIIIIII IOK RESISTIVITY OHM/SQ/MILL Illll I l INVEP-JTOR. SCHEBALI N SERGEI BY WMQ AGENT.

Jan. 29, 1974 SCHEBAL'" 3,788,891

THICK FILM PRECISION RESISTOR FOR USE IN AN ELECTRICAL CIRCUIT ANDMETHOD OF MAKING SAME Filed March 2, 1971 5 Sheets-Sheet 4.

FIG. 8

DEPENDENCE OF TCR ON THE ASPECT RATIO (GEOMETRY) OF A RESISTOR I30 PPM/c ZOPPMOC TCR OF A RESISTOR IN PPM/C III I I I I I I I I j .|.s| 2 a 4 5e a 9 ASPECT RATIO(GEOMETRY) OF A RESISTOR INVENTOR. SERGEI SCI-IEBALINJan. 29, 1974 Filed March 2 RESISTIVITY Il/El/MILL scHEBALm 3,788,891THICK FILM PRECISION RESISTOR FOR USE IN AN ELECTRICAL CIRCUIT ANDMETHOD OF MAKING SAME 1971 5 Sheets-Sheet 5 FIG. 9

DEPENDENCE OF RESISTIVITY ON ASPECT RATIO (GEOMETRY) OF A RESISTOR BllOOIL/U/MILL ZISOIL/El/MILL IISOJL/El/MILL ASPECT RATIO INVENTOR.SCHEBALI N WWW SERGE! AGENT.

3,788,891 THICK FILM PRECISION RESISTOR FOR USE IN AN ELECTRICAL CIRCUITAND METHOD OF MAKING SAME Sergei Schebalin, Ambler, Pa., assignor toHoneywell Inc., Minneapolis, Minn. Filed Mar. 2, 1971, Ser. No. 120,199Int. Cl. H01c 7/00 US. Cl. 117-212 12 Claims ABSTRACT OF THE DISCLOSUREA thick film resistor is formed by #1 firing selected mixtures ofpositive and negative thick film resistor ink materials on anon-electrically conductive substrate at a selected high temperature and#2 subsequently jointly firing the resistor and conductor ink materialassociated with this resistor, at a temperature that is lower than thefirst mentioned temperature and which is at a level that will not allowany detrimental diffusion, to occur between the conductor and theresistor materials. This unique process of firing achieves for the firsttime a successful method of firing a resistor and conductor jointlywithout allowing any adverse change to occur in the value of thetemperature coefiicient of resistivity and the resistivity value of theresistor.

It is an object of the present invention to provide a unique precisionthick film, cermet, resistor and a unique method that can be employed tomanufacture this type of resistor.

It is an other object of the present invention to provide a resistor ofthe aforementioned type that possesses electrical resistancecharacteristics, that are precise, and whose performance is unaifectedby the time it remains on the shelf, the time period over which it isemployed in an electrical circuit and changes in ambient temperature.

More specifically, it is another object of the invention to provide aprecision resistor of the aforementioned type whose resistance willremain within an acceptable i0.l% level over long periods of use thatextend beyond a two year period of time.

It is another object of the invention to provide a method ofmanufacturing a cermet resistor for use in measuring circuits wholeaccuracy and overall stability is as good and reliable as thosepossessed by present day commercially available wire wound resistors.

One of the terms that is used to define a critical characteristic of athick film resistor is its sheet resistivity. This term sheetresistivity relates to the electrical resistance which a one milli-inchthick square of any size of resistive material offers to a steadycurrent passing between any two opposite faces of this resistivematerial along which for example a conductive film is attached. Thissheet resistivity is known to vary with ambient temperature betweene.g., +300 p.p.m./ C. to 300 p.p.m./ C. depending on the sheetresistivity of resistor material being used.

Another term that is used to define the characteristic of a thick filmresistor is TCR or temperature coeflicient of resistivity which is thechange in resistivity expressed in ohms per degree centigrade.

In achieving the aforementioned objectives it has been discovered thatan adverse change in resistivity and TCR of a thick film cermet resistoris caused by diffusion of the conductive material in the conductor,which has an extremely low resistivity value, into resistive material ofthe resistor which has a much greater resistance value when the resistorand conductor are fired on a ceramic substrate. Heretofore it was acommon belief that this adverse change was based upon the geometry, orthe so- United States Patent 'ice called aspect ratio factor which is aratio of the length to width of the resistor.

It is another object of the invention to recognize for the first timethat the aforementioned detrimental effect of diffusion is much greaterbetween the ends of a rectangular strip of resistor and a conductor thatextends away from the resistor when the longest opposite sides of therectangular resistor strip are selected for connection to the conductorfor jointly firing onto a substrate rather than the shorter oppositesides of the rectangular resistor strip.

Furthermore, experimentation has shown that firing temperature changesadversely affects TCR and the resulting resistivity of thick filmresistors because of the high degree of the aforementioned difiusionthat takes place between the resistor and the associated conductors towhich it is attached when they are jointly fired.

It is therefore another object of the invention to provide a uniquemethod of firing resistor and conductor inks onto substrates so that noundesired diffusion will take place between the resistive and conductivematerials that will adversely effect the TCR and resistivity andtherefore the precise resistance offered by the resistor.

To accomplish the aforementioned feat it is another object of theinvention to provide a means whereby the dried resistor ink is firstfired for a preferred preselected period of time e.g., 15 minutes at ahigh temperature of e.g., 1000 C. on a substrate to form an amorphicmass and thereafter the conductor extending from either side of theresistor is printed, dried and then fired for a similar period of timeat a substantially lower temperature in the neighborhood of 550 C., ontothe already fired resistor to eliminate substantially all of theundesired diffusion of the conductive material that would otherwisedifiuse into or from the resistor material.

It is another object of the invention to provide a method of theaforementioned type which will allow an ink, such as a resistor inkhaving a high firing temperature to be fired at a high temperature ontoa substrate, and a conductor ink having a lower firing temperature thanthe resistor ink to be then fired jointly with portions of the alreadyfired resistor ink onto the substrate so that undesired diffusion of theconductor ink material into the fired resistor ink will be negligibleand an acceptable cermet resistor having a low TCR to be produced.

Heretofore it was a common practice, after the selecting the size andthe aspect ratio of a resistor which would fall within a prescribedresistance range to consult a table or a graph filled with geometrycorrecting tables that predicts the resistivity and the TCR as afunction of the resistors geometry. This procedure is slow, tedious, andhas circuit design limitations in that the resistor could only be madeof a certain geometric shape and the size. It is well known that theseresistors will have different TCR and resistivity values.

It is therefore another object of the invention to eliminate the needfor the aforementioned tables.

It is also another object to provide a method of manufacturing a cermetresistor whose shape can 'be of any one of a number of difierent formsor configurations, and need not therefore be limited to a restrictedshape as has heretofore been required.

It is another object of the invention to provide a com struction for acermet resistor that will present its resistivity and TCR values frombeing changed by the destructive oxidating or other detrimental effectsresulting from the resistors exposure to air, moisture, hydrogen sulfideor other similar ambient atmospheres.

It is another object of the invention to provide a method of blendingone or more positive TCR resistor inks with one or more negative TCRresistor inks so that the resulting temperature coefficient ofresistivity TCR and the resistivity of the resulting resistor can beprecisely predicted by changing the blending proportions of the negativeTCR resistor ink and the positive TCR resistor ink before firing in theaforementioned unique manner so that a number of different shapedresistors can be formed which individually possess different preciselyfixed resistance values.

It is another object to provide resistors of the aforementioned thatextend -over a Wide range and which will result in each of the resistorshaving a temperature coefiicient of resistivity value, TCR, that iswithin a few parts per million per degrees centigrade from zero.

Since it is not possible to obtain a precise resistance value for theresistor from the aforementioned unique firing process nor from anyother firing process it is therefore another object of the presentinvention to provide a means of trimming such a resistor after it hasbeen fired so that a more exact value of the resistance can be achievedfor these resistors.

A better understanding of the present invention may be had from thefollowing detailed description when read in connection with theaccompanying drawings in which:

FIG. 1 shows a nomograph having a uniquely constructed semi-log scalefor graphically determining the amount of additional positive ornegative ink that should be added to an ink mixture of positive andnegative inks to provide a thick film resistor ink of a desired zeroTCR;

FIG. 2 shows the steps required in a first method of trimming theaforementioned thick film resistor;

FIG. 3 shows the first step required in a second method of trimming theaforementioned thick film resistor;

FIG. 4 shows the second step required in the second method of trimming athick film resistor;

FIG. 5 shows the third step required in the second method of trimmingthe aforementioned thick film resistor;

FIG. 6 shows how the aforementioned trimmed thick film resistor can beencapsulated to prevent the destructive oxidating effect of the ambientatmosphere from effecting its resistivity and TCR value; and

FIG. 7 shows a chart having a solid line thereon to indicate the wideresistivity range of values over which a zero TCR prevails for manydifferent positive and negative cermet resistor ink blends when they areproduced by the unique method to be hereinafter described in which nodiffusion is allowed to occur between the conductor and the resistor ascontrasted by the line shown in dash line thereon which indicates thatzero TCR can be achieved for only a single resistivity value of manypositive and negative cermet resistor inks when they are produced by thewell known profile firing method as a result of undesired diffusionoccurring between the conductor and its associated resistor.

FIG. 8 is a graph to vividly illustrate the desirable independentrelationship that can be achieved, as shown by curve B, between thetemperature coefiicient of resistivity and the aspect ratio [geometry]values of thick film resistors by firing them in the previously referredto unique non-diffused maner with their associated conductors onto asubstrate. FIG. 8 also shows a curve A which represents the undesireddependent, restricted, temperature coefficient of resistivity versusaspect ratio [geometry] relationship that must be adhered to when thickfilm resistors and their associated conductors are fired jointly at ahigh temperature Which causes diffusion to occur be tween the lastmentioned conductors and their associated resistors.

FIG. 9 is a graph to vividly illustrate the desirable independentrelationship that can be achieved as shown by curve B between theresistivity and the aspect ratio [geometry] values of thick filmresistors by firing them in the previously referred to uniquenon-diffused manner with their associated conductors, in a non-diffusedmanner, onto a substrate. FIG. 9 also shows a curve A which representsthe undesirable dependent restricted resistivity versus aspect ratio[geometry] relationship that must be adhered to when thick filmresistors and the associated conductors are fired jointly at a hightemperature which causes diffusion to occur between the last mentionedconductors and there associated resistors.

Method of blending cermet inks to fabricate thick film resistors whichare not sensitive to temperature changes.

The temperature coefficient of resistivity, TCR, for thick film cermetresistors has heretofore been changed by altering the firing temperatureprofile and/or by changing the geometry, or in other words thepreviously referred to aspect ratio of these resistors.

Since the changes in TCR obtained by these methods are several parts permillion per degree centigrade, p.p. m./ 0., usually in the vicinity of 1to 10 p.p.m./ C. for one degree C. change in firing temperature, theyare therefore not sufficiently exact to obtain the desired TCR value.

A unique method of ink blending to obtain a desired TCR value which doesnot have to rely on the selection of a desired firing temperatureprofile will now be described.

The magnitude of change of TCR obtained by this unique method is atleast 10 times larger than the previously mentioned method which wasbased upon a change in firing profile and a change in the geometry ofthe resistor.

Experimentation has shown that an addition of a metal in powder formsuch as a gold powder with the particle size of three to twenty micronsor a metal powder mixed with lead-boro-silicate glass powder of the sameparticle size when mixed with a liquid agent such as decanol provide asuspension that will decrease the sheet resistivity of a resistor andcause a change in its TCR in a positive direction. The addition of metaloxide powder, for example ruthenium oxide powder, or 'a metal oxidepowder mixed with boro-lead-silicate glass powder and a liquid agentsuch as decanol causes a change in TOR in a negative direction. It cantherefore be concluded that by adding metal or metal oxide to a cermetink the TCR is changed in either a desired positive or a negativedirection. And, therefore, if two or more resistor inks are availableand if one of them has a positive TCR and the other a negative TCR orvice versa, they may be blended to obtain a desired TCR, and theblending proportion can be calculated by the method to be hereinafterdescribed:

Measure the TCR of resistors made from the ink which is to be modifiedto obtain a zero TCR resistor. This measurement of TCR is accomplishedby firing the resistor ink on an electrically non-conductive substrateand then taking measurements of its electrical resistance at roomtemperature such as 73 F. and at a higher temperature such as 173 F. andcalculating the TCR from these values by the following formula TCR=where AR equals the resistance of the resistor at the aforementionedhigh temperature minus its resistance at the aforementioned roomtemperature.

R is the resistance value at room temperature and At equals thedifference between the aforementioned high temperature and roomtemperature.

If the TCR value of the resistor is zero no further modification of theink is needed. If it is not zero and it is negative, then a metal suchas gold is added to the ink. It is then blended and a measurement of itsTCR value is again made in a manner similar to that already described.

The amount of metal added to the ink, such as gold must be large enoughto provide a positive TCR value of not less than 20 parts per millionper degrees centigrade. If the TCR of the ink under modification isfound to be positive then metal oxide, e.g. powder 325 mesh, such asruthenium oxide, is added until the ink provides a negative TCR resistormaterial of 20 parts per million per degrees centigrade or a highernegative number. The purpose of the above modification of the availablecommercial inks is to make a pair of inks so that one of the pair willhave a negative TCR resistor value and the other of the same pair willhave a positive TCR value. These two inks are then blended by mixingthem together in a proportion that will provide a blend of zero TCR ink.The amount of positive TCR ink and the negative TCR ink forming theblended proportion is calculated from the following equation and is doneas explained below:

P=Exp Where The percentage of positive ink in a blend which will providezero TCR resistors is found through the use of the aforementionedequation. This same percentage can also be found graphically by firstplotting the TCR of the positive ink on the semi-log paper chart asshown in FIG. 1. The point T plotted on the semi-log chart shown in FIG.1 is the TCR of a positive ink or in other words is the TCR of a blendwhich consists of 100% positive ink. The abscissa of this point does notcorrespond with the 100% point on the abscissa axis but insteadpurposefully corresponds with the 103% point. This offset of 3% is thevariable S in the aforementioned equation. In this particular examplewhere ruthenium system ink is used it has been found by experimentationthat the value of 8:3.

The TCR of a negative TCR ink is then plotted as an ordinate on a linearscale on the semi-log chart of FIG. 1 as the point T This represents thevalue of a blend which has zero percent of positive ink in it. Theabscissa, or log scale value, of this point does not correspond with thezero percent point on the abscissa axis, but rather corresponds with the3% point selected as a result of statistical data derived fromexperimentation. This offset of 3% is variable S in the equation. Afterthe positive and negative TCRs of a pair of inks are plotted asdescribed above the percentage of positive ink P which should be in theblend to provide zero TCR resistors is found as follows:

A straight line is drawn between T and T This line represents a changein- TCR of resistors versus percentage of positive TCR ink in the blendand crosses the zero TCR line. Looking at the base of the graphimmediately below the point at which the aforementioned line crosses thezero TCR line we [find that its value as read on the abscissa is thepercentage of positive ink in the blend which will provide desired zeroTCR resistor value. It should be noted that the value of this pointalong the abscissa is the value of the P shown in the previouslymentioned equation.

Knowing the percentage P of the positive ink in the blend the zero TORblend can then be prepared. However the TCR of the resistors made fromthis blend will not necessarily be zero it may not even be within thezero plus or minus 20 parts per million per degree centigrade limits.This is so because the previously mentioned equation represents the bestfit or linearized condition that can be derived from the TCR versus logpercentage of blend that exists for several different blendingproportions. The degree of misfit depends on the number of test blendsand on the ink composition. If the TCR of resistors prepared from thisblend is not zero as calculated from the previously mentioned equationAR o TCR= 10 in p.p.m./ C.

or is not within the desired limits, the blend must be corrected. It isevident that the process parameters relating to the preparation ofresistors must be constant. For example the firing profile, absolutehumidity in the furnace, atmosphere in the furnace, and the driedthickness of the resistors must be kept constant.

The following blending proportion correction is preformed in order tobring the TCR of the resistor closer to a zero value.

The actual value of the TCR of the resistor, TCR as derived from theequations TORF 6 Ate X 10 is determined from representative samples ofthe blend and is plotted in FIG. 1. If this TCR is positive as indicatedby its plotted position in FIG. 1 this point TCR is connected with thealready plotted point T or in other words, the point which is the TCRvalue of the negative TCR ink. This line between the points TCR and Trepresents a corrected change in TCR of resistors vs percentage ofpositive TCR ink in the blend or in other words the change in TCR ofresistors vs percentage of positive TCR ink in the blend which waspreviously determined in FIG. 1 was incorrect due to imperfectlinearization when parameters were chosen as previously described forthe first previously mentioned equation that was used to figure out thevalue of P.

If this TCR were negative e.g., TCR this TCR point would be connected bya straight line to the point T The line connecting point TCR with pointT or TCR with point T must in each instance cross the zero TCR line. Inone example the TCR of the resistors made from a blend prepared by thepreviously mentioned graphical method is positive and plotted at itspoint TCR in FIG. 1. The abscissa of the point of intersection or pointI on FIG. 1 between the TCR -T line and the zero TCR line is a correctedpercentage of positive ink in the blend which shall provide zero TCRresistors and is marked on FIG. 1 as P Knowing P which is the correctedand more accurate percentage of positive TCR ink in the blend, the blendcan be either corrected by adding corresponding amounts of negative TCRink to the blend or a new second blend can be prepared based on theinformation derived in the aforementioned manner.

Even now, the second corrected blend may still not provide zero TCRresistors. If this is the case a second correction is needed and the TCRof the resistors made from #2 blend as determined from the equation inFIG. 1 as point TCR In this example TCR turned out to be negative. Aline is drawn through this point TCR and the previously obtained pointTCR This line represents the second corrected change of TCR vspercentage of positive ink in the blend. The abscissa of the point ofintersection between the line TCR -TCR and the zero TCR line is acorrected percentage of positive inks in the blend which will providezero TCR resistors and is marked P in FIG. 1.

Knowing P which is the second corrected percentage of positive TCR inkin the blend, this blend can then be either corrected by addingcorresponding amounts of positive TCR ink for example ink with TCR==T ora new third blend can be prepared based on the aforementionedinformation.

In the above example the TCR of the second blend TCR was negative. If itwere positive then the TCR point would be connected by a straight linewith point T and the abscissa of the point of intersection between lineTCR T and the zero TCR line would be the percentage of positive TCR inkin the third blend. When addition blend of the correction of the blendis needed, such as in the case where the TCR of the resistors made fromthe blend are outside of the desired limits, the last obtained andplotted TCR point e.g., TCR is then connected with the nearest TCR pointof opposite sign as measured along the abscissa. The abscissa of theintersection point of this last mentioned line which connects the twonearest TCR points of opposite signs with the zero TCR line representsthe percentage P of positive TCR ink which should be in the correctedblend.

Experimentation has shown that in the majority of blending operationsonly two such corrections are sufficient to bring the TCR within the $20parts per million per centigrade limits.

It should also be further understood that a method has been describedthat can be used for obtaining any desired TCR for resistors other thanzero by observing where the interconnecting line between T and T passesa horizontal line on the chart that passes through the desired positiveor negative value of the blend that is desired rather than through theother line. This TCR of the blend cannot of course be made more negativeor more positive than the TCR value of the two basic inks that were usedto make this blend.

The change in TCR resistors causes the change in the sheet resistivityof the resistors and the more negative that the TCR is the higher willbe the sheet resistivity. This is so because the addition of metallicoxide to the ink causes the TCR to change in the negative direction andincreases the sheets resistivity.

Knowing the sheet resistivity of the two inks which are used forblending and knowing their percentage in the final blend the sheetresistivity of resistors made from this blend can be easily predicted byusing kown methods of calculation.

Methods of trimming of high accuracy resistors (cermet) with lowaccuracy trimming machine Present day accuracy of cermet resistors afterthey are printed and fired is about 20% of the value desired. Therefore,if a better accuracy is desired, they must be corrected. Usually, thecorrection consists in removing a portion of the resistor until theresistance reaches the desired value. A partial removal of resistormaterial causes an increase in the resistance. In other words the valueon resistance can be corrected only in the direction of increase of theresistance.

Usually the resistance of the resistor under trimming is constantlymeasured by usually a high precision resistance measuring bridge forexample, a Kelvin bridge.

The accuracy of resistance measuring bridges is usually of i0.05%.However the accuracy of trimmed resistors is seldom better than 11%.This is caused by unpredictable time lag between the electrical signalfrom the bidge, indicating that the resistor has reached its desiredvalue and the execution of this signal (i.e., stopping the trimming) byconventional electromechanical and pneumatic links between the bridgeand the cutting device. The degree of overtrim or, in other words,over-cuts, depends on the speed of the cutting device which is usually anozzle which directs the stream of abrasive particles on the resistorand also on the resistivity of the resistorss material.

The higher the nozzle speed and the resistivity, the larger 8 will beovertrim or error. Usually the degree of overtrim does not exceed 1% ofnominal desired resistance. In other words, even if the trimmingmachine, which includes resistance measuring bridge and the cuttingdevices, has a high precision bridge, its total accuracy, i.e., theaccuracy of trim, usually is in a low precision range.

Described below are two methods of trimming a high precision resistor 10with low precision trimming machines, which have high precisionresistance measuring bridge. The first method is as follows:

The resistor 10 is laid out so that it consists of two parts 12 and 14as shown on FIG. 2. Part 12 measured between points a and b ofconductive parts 16, 18 in FIG. 2 must be of sufiicient size, length onFIG. 2, to provide at least 98% of the nominal desired total resistanceafter trimming along trimming path 20. Part 14 resistor measured betweenthe points b and c of conductors 18, 22 must have not more than 1% ofthe nominal total resistance before "it is trimmed along the trimmingpath 24.

Measuring across the entire resistor i.e., between the points a and c ofconductors 16, 22, the resistor part 12 is trimmed to 98% of the totalnominal resistance because the accuracy of trimming machine is :1%. Theresistance of the entire resistor measured between a and 0 will be98%-31% of the total nominal resistance and the resistance of justtrimmed resistor alone measured between a and b of conductors 16 and 22can be 98% of nominal :l% of nominal -R,,,,, where R is the resist anceof yet untrimmed part 10 measured between 18 and 22.

As it was mentioned before, the maximum resistance of untrimmed Rresistor 10 does not exceed 1% of total nominal resistance, therefore,in the worst case, the minimum resistance of just trimmed R resistor is98%- 1% =97% of the total nominal resistance.

To correct the error after the first trim along trim path 20 the actualvalue of the trimmed R resistor part 12 must be measured. Since theresistance measuring bridge only is involved in this measurement, themeasured value of R resistor 12 will be within the accuracy of thebridge i.e., usually within 10.05%. This uncertainty in the trimmed Rvalue can obviously not be corrected, and it depends on the accuracy ofmeasuring bridge alone.

Assuming that R resistance of resistor part 12 after trim was 97% oftotal nominal resistance, the yet untrimmed resistor 14 must be trimmedalong trimming path 24 until it reaches 100% -97%'=3% of the totalnominal value.

Because of :1% accuracy of the tirmming machine, the value of trimmedresistor 14 measured between points b and c during the trimming, andtrimmed along trim path 24 to 3% of total nominal resistance will be 3%of nominal i-l% of 3% of nominal or 3% i.03% of total nominal value vs.desired 3% of nominal. Assuming one of the worst possible cases, theresistance of trimmed resistor part 14 can be 3%.03%-=2.97% of the totalnominal value; and the total value will be R (part 12) +R (part14)=97%-+2.97%'=99.97% of the total nominal value, ie the error aftertwo trims will be -.03%. Adding the uncertainty or the resistancemeasuring bridge (1.05%) the maximum error after two trims will be (inthis example) i.05.02=-.08%.

The above example shows that, dividing the resistor 10 into two parts12, 14 and performing two trims 20, 24, the final accuracy obtained ismore than 10 times better than the accuracy of conventional trimmingmachine and that it approached the accuracy of a precision resistancemeasuring bridge.

The accuracy obtainable by this two trim method is determined asfollows:

R=Nominal (desired) value of R (Ohms) C=Accuracy of resistance measuringbridge (percent) A=Accuracy of trimming machine (including percentbridge accuracy) B=Value of R untrimmed (in percent of nominal) R =part1 of total resistor R (or 'R resistor (9) R =part 2 of total resistor R(or R resistor (n) R =Total resistance (R =R +R (Q) E=Total maximumerror in R after trimming (in percent of nominal) After 1st trimdescribed above:

R =(1-A)R -t(l)RA Note: The resistor is trimmed to (l-A) percent ofnominal value. A is expressed in decimals.

Note: B is expressed in decimals Note: C is expressed in decimals theerror in R trimmed is measured with res. bridge of :C percent accuracy.

After the second trim i.e., after R is trimm d to 1 trlmmed ]|V3.l.l1eZ

max error in R R Note: A, B, C, expressed in percent A numerical examplewhere as it was described before will yield the following accuracy inthe resistor trimmed by described method:

Max. error=i(2(.01% +.0l% +.000l% A second method of trimming highprecision resistors with low precision trimming machine (with highprecision bridge) is described below, and shown on FIGS. 2, 3, and 4.

The resistor 10 is trimmed along a trimming path as shown at 26 to 98%of its desired value. The maximum error after this trim is usually i lWithout changing the resistor position in the trimming machine, theresistor 10 is trimmed again to 99.5% of the desired value alongtrimming path 28. That is the cutting device (which usually is a nozzlewhich provides a jet of abrasive particles suspended in air) repeats thesame cutting pattern. Exeriments have shown that the amount of resistorsmaterial removed by this second trimming is about 0.5% of that removedin the first trimming. This Percent max error in R 10 is equivalent toslowing down the trimming speed by the factor of The same trimmingpattern is repeated for a third time along trimming path 30 and theresistor is trimmed to its desired value. The amount of resistormaterial removed by this third trim is about .05% of that removed by thefirst trim, which is equivalent to slowing down the trimming speed bythe factor of V1000 as compared with the first trimming speed.

The accuracy of the trimmed resistors (assuming the accuracy ofresistance measuring bridge as i-.0S%) is usually in the order ofODS-0.09%, which is comparable with the first described method.

The accuracy of trimmed resistors can be improved further if four trimsare used instead of three, approaching the accuracy of the resistancemeasuring bridge.

Both of the methods described herein allow a single thick film resistorto be trimmed to any one of a number of desired values.

The aforementioned precise trimming method enables a reduction to bemade in the cost of manufacturing resistors having different resistorvalues because the same common blend of positive and negative resistorink having a 0 TCR can be fired onto each one of a number of substratesbefore different individual selective trimming of each of theseresistors occurs.

It should be noted that trimming of the cermet resistor by either of theaforementioned methods is done after the previously described selectedzero T CR blend of positive and negative cermet resistor ink that wasused to form resistor 10 has been fired onto the aluminum oxidesubstrate 32.

When a thick film cermet resistor 10 of the aforementioned type is leftexposed to its surrounding atmosphere its precisely manufacturedresistance and TCR value will be altered with time because of thedestructive oxidation and other similar detrimental effects which air,moisture, hydrogen sulfide or other similar destructive delequescentmaterials have on the resistor 10.

More particularly the stability of cermet resistors that are notprotected from the ambient atmosphere, whether under a load or no loadcondition, is usually in the order of 03-05% per year. In other words,the resistance of these resistors have heretofore changed by 0.3 %-0.5 ayear after they are manufactured.

Such a poor stability precludes the possibility of the manufacture ofhigh precision resistors which have a tolerance of i-O.l% or better.

Manufacturing a thick film resistor in the manner to be hereinafterdescribed provides a resistor which will retain a stability of 0.1% forat least two years.

In other words the resistance of these resistors will change no morethan 0.1% of their nominal value after two years of active use in acircuit or during the period in which they are stored on the shelf forthis length of time.

Experimental tests showed that the main reason for the instability ofthick film resistors, or in other words, drift in resistivity with timewas caused by oxidation of the metals in the resistor and by absor btionof water, contained in the atmosphere.

Therefore, the resistors must be insulated from the ambient atmosphere.

To solve this problem an insulator layer must be provided which has thesame temperature coefficient of expansion as the ceramic substrate 32and the resistor 10 and conductor 16 and 22. Otherwise thermal stresseswill develop with an accompanying change in resistance. Another way isto make the insulating layer flexible enough to prevent stresses fromoccurring in the resistor which would change its resistance by more than:.05% for the desired specified temperature range e.g., a change inambient temperature. Also the layer which physically contacts theresistor must be chemically inert with respect to resistor material, forexample, it should not cause oxidation or reduction of the resistormaterial to occur and at the same time it must be able to adhere to theresistor 10. Another factor that had to be considered was that since theflexible insulation layer must possess a soft flexible characteristic itneeds additional protection from mechanical damage such as scratchesetc. The encapsulating structure on the substrate as described belowprovides a system of layers to protect the cermet resistor from ambientair, water vapors, water, and hydrogen sulfide (I-1 S). Furthermore thelayers to be described have been found satisfactory in protecting theresistor from being effected when continuous changes in ambienttemperature that may vary from the standard reference level of 25 C.:50C. so that no more than 1.1% change in value of the resistor can atleast occur.

The ceramic substrate 32 which is preferably a 96% pure aluminum oxidematerial is exposed to 1000 C. for to minutes. It is assumed that thesubstrate 32 is clean prior to this operation if it is not it is cleanedultrasonically in ethyl or methyl alcohol for three minutes. Next thepreviously mentioned resistor 10 and conductor inks 16, 22 are printed,dried and fired in the manner previously described as shown in FIG. 6-0f the drawing.

A silicon polymer filled with magnesium oxide 34 such asdimethylpolyxylene which is commercially available from the EMCA Companyas plastic coat 1139B is then printed or brushed over the resistor 10and conductor areas 16, 22 except for the conductor areas that arereserved for the terminals 38 and 40. The substrate 32 is then heatcured at 108 C. for 24 hours to provide polymerization and the resistor10 is trimmed through the plastic coat 34 to 99% :1% of its desiredvalue as previously described and the terminals 38, 40 are then solderedwith suitable soldering material 42, 44 as shown in FIG. 6.

Next, the substrate 32 is heat cycled twice between C. to 125 C. at thetemperature-time slope of 20 C. per minute and kept for 2 hours at 125C. then cooled down on the same rate to 25 C. The same heat cycle isrepeated for a second time, and then for a third time a period offifteen hours instead of two hours and at the same temperatures.

The resistor is then finally trimmed as previously described under thedescription of FIGS. 2-5 to minus .03% of the desired value. Theresistor is then cleaned with a jet spray of nitrogen. A mixture of ironoxide with magnesium silicate suspended in xylene 36 such as glyptal1201B paint that is commercially available is aprayed over the entiresubstrate including the resistor conductors and portions which form thesolder joint and terminals. The substrate is then exposed to 100 C. forfour hours.

The thickness of the flexible silicone polymer layer 34 that is selectedis never less than twelve microns and the thickness of the hard glyptallayer 36 is not less than fifty microns. A cross sectional view of theprojected resistor 10 is as shown in FIG. 6.

Experimentation has also shown that cermet resistors that are preparedin the above-described manner will remain stable within 1.1% for atleast two years or more.

The plotted dotted line shown in FIG. 7 indicates that it is possiblethrough the use of a conventional diffusion introducing profile firingmethod to obtain only a single resistor blend of ink that has a zero TCRvalue from a series of different blends of inks which possess differentsheet resistivity values.

FIG. 7 also shows a second plotted solid line to indicate that a seriesof resistors having different resistivity values over a wide resistivityrange can be obtained which each has a 0 TCR value when the resistor isfirst fired by the unique non diffusing method previously described inwhich the resistor is first fired to the substrate at one temperatureand the conductor and resistor are thereafter jointly fired at a secondtemperature that is 12 approximately 500 C. lower than the firstmentioned temperature.

The unique steps employed in the preparation of a zero- TKIR thick filmresistor are:

(1) Ultrasonically clean substrate 32 in methanol for thirty seconds.

(2) Prefire substrate 32 at 1,000 C.

(3) Clean substrate 32 with N print resistor.

(4) Dry resistor at 107 C. for 45 minutes.

(5) Ascertain the correct firing temperature of the fur nace.

(6) Fire resistor at a plateau temperature of 1,000 C.

on a two inch per minute moving belt.

(7) Clean substrate 32 with N and print conductors (8) Dry conductors at107 C. for 45 minutes.

(9) Ascertain the correct firing temperature of the furnace.

(10) Fire the conductors at a plateau temperature of 550 C. on a 2" perminute moving belt.

(11) Anneal by heat cycling at 177 C. for 15 hours.

(12) Clean resistor 10 and conductors 16, 22 with N and screen onflexible layer 34.

(13) Dry flexible layer 34 at 126 C. for 12 hours.

(14) Stake pins 38 and 40 and solder at 42, 44.

(15) Trim resistor 10 to ninty eight percent of its normal resistancevalue and clean With N (16) Heat cycle at 121 C. two times for two hoursand then overnight to eliminate stresses induced by trimming.

(17) Trim to desired value and clean with N (18) Spray on hard layer 36.

(19) Dry hard layer 36 at 93 C. for 4 hours.

The significance of eliminating the harmful effects the diffusion has onTCR and resistivity which has heretofore been brought about by firingthe resistor and conductor at substantially the same high temperature isclearly illustrated in FIGS. 8 and 9.

FIGS. 8 and 9 show for example how TCR and the resistivity of thick filmresistors are dependent on the previously referred to geometry, oraspect ratio of the resistor when they are fired with associatedconductors at the same high temperature and how this dependence waspractically eliminated when the unique process heretofore described wasemployed.

Curve A in FIG. 8 shows the dependence of TCR on the aspect ratio when athick film precision resistor is manufactured by using conventionalmethods in which the resistor and conductor is fired at the same hightemperature. It can be seen in this conventional method that the TCRvalue changed from {+74 p.p.m./degree C. at the aspect ratio of .1 to 56p.p.m./degree C., at the aspect ratio of 10. In other words curve Ashows that a total change in TCR of p.p.m./degree C. occurred when thepreviously mentioned ruthenium system ink is used as the resistormaterial, platinum gold ink is used as the conductor material and afterthe resistor ink and conductor were fired at the high temperature of1000 C.

FIG. 8 curve B shows how the dependence of TCR on the aspect ratio isfor all practical purposes eliminated when the thick film resistor ismanufactured by the previously described unique method of manufacturing.

The dependence of TCR on the aspect ratio decreases from 130 p.p.m. perdegree C. for conventional methods that have heretofore been used asshown in FIG. 8 curve A to 28 p.p.m. per degree C. for the unique methodof manufacturing that has for the first time been disclosed herein.

Furthermore, in addition to the decrease in the TCR dependence on theaspect ratio it can be seen that the TCR versus aspect ratio curveshifts in a negative direction after the unique manufacturing method wasused.

More specifically both curves A and B represented the relationshipbetween the TCR and the aspect ratio for the same ruthenium system inkresistor material. The only difference in the manufacturing processdepicted in the curve A and B shown in FIG. 8 is that in curve A theresistors and their associated conductors were fired at approximatelythe same temperature such as about 1000 C. and for curve B the resistorswere first fired at 1000 C. and thereafter the resistors and theirassociated conductors were jointly fired at about 550 C.

It should also be noted that the conductor connected the resistorrepresented by curve B contains silver combined with glass instead ofthe conventional platinum-gold (PdAn) combined with glass type conductoras represented by curve A. The only reason for the change in conductormaterial from Pd Au to silver was that it was not possible to fire a PdAu type conductor at 550 C. whereas a silver type conductor can be firedat this temperature.

A shift of curve B in a negative direction shows that the degree ofdiffusion of conductor material into the resistor material has decreasedand that the true TCR of the resistor ink is in reality that shown oncurve B rather than the generally heretofore assumed value that is shownon curve A.

Curve C, FIG. 8 depicts the dependence of TCR on the aspect ratio afterthe resistive ink was blended with ink having positive TCR such as Pt Autype conductor ink or pure gold powder. After blending the resulting inklies approximately between +13 p.p.m./degree C. and 8 p.p.m. degree C.TCR and its dependence on the geometry (aspect ratio) for all practicalpurposes is nil.

FIG. 9, curve A shows the dependence of resistivity on the aspect ratiowhen the thick film resistors are manufactured by conventional methods.

2150 ohm per square per mil was the change in resistivity that occurredwhen a change in aspect ratio went from .1 to 10.

Curve B of FIG. 9 provides a way of showing the independence ofresistivity on the aspect ratio when the resistor is manufactured by theunique method previously described for FIG. 8, curve B.

The dependence of resistivity on the aspect ratio [geometry] decreasesfrom 2150 ohm per square per mil on curve A to 1100 ohm per square permil on curve B.

FIG. 9 curve C shOWs the resistivity versus aspect ratio after blendingas previously described under FIG. 80.

It has been determined by experimentation that substantially 20% byweight of RuO 40% by weight of Ru and 40% by weight of glass frit is onetype of positive temperature coelficient of resistivity resistor inkthat can be employed to advantage in the aforementioned described inkmixtures that are formed from positive and negative temperaturecoefiicient of resistors ink. It has also been determined by experimentthat substantially 40% by weight of Ru O 20% by weight of Ru and 40% byweight of glass frit is one type of negative temperature coeflicientresistivity resistor ink that can be advantageously used in theaforementioned ink mixture to make the resistor 10.

It can therefore be seen that the unique apparatus and method offirst'firing the resistor 10 onto a substrate 32 at 1000 C. and thelater joint firing of the resistor 10 and its associated conductors 16,22 onto the substrate 32 at a lower temperature, namely 550 C., willsubstantially eliminate diffusion that has heretofore occurred betweenthe conductor material and the resistor material.

By procuring a resistor and its associated conductors after firing inthe substantially same undilfused state that they were in before firingit is for the first time possible to eliminate the TCR and resistivitydependency on the geometry of the resistor commonly referred to asaspect ratio that has heretofore existed when other firing means havebeen employed for this purpose.

Because of the aforementioned advantages derived from the unique firingtechnique it is now possible for the time to manufacture precision thickfilm resistors with- 14 out concerning oneself with the heretoforeexisting problem of:

1) Selecting the right aspect ratio or in other words the length-wideratio, or the geometry, of the resistor that is to be fired onto asubstrate.

(2) Spending time in consulting geometry correcting tables to predictresistivity and the TCR of the resistor as a function of the resistorsgeometry and (3) Requiring the creative ability of the designer who isdesigning an electrical thick film circuit from being able to presentthe most desired economical compact circuit because the resistors thathave heretofore been used were required to be of a prescribed geometricshape in size.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A process for firing a precision thick film resistor and anassociated partially overlapping conductive material on an electricallynonconductive substrate in such a manner that substantially none of theconductive material is diffused into the resistor whereby thetemperature coefiicient of resistivity of the resistor will have a valuethat is within a few parts per million per degrees centigrade from zero,said resistor further being characterized that the said temperaturecoeflicient of resistivity and sheet resistivity thereof is free fromdependence upon the geometric shape thereof, said process comprising thesteps of applying a resistor ink to the substrate, drying the resistorink, firing the resistor ink in amorphous form to the substrate atthesintering temperature of the resistor ink, applying the conductivematerial to the substrate in partially overlapping relationship withsaid fired resistor, drying the conductive material, firing theconductive material at a sintering temperature which is substantiallylower than the first mentioned firing temperature whereby the tendencyfor diffusion of said conductive material into said fired resistor issubstantially prohibited.

2. The process as defined in claim 1 wherein the resistor containsdifferent combinations of positive and negative resistor ink mixes andthe conductor film is constructed of substantially seventy percent byweight of silver powder, substantially ten percent by weight of leadboro silicate glass and the remainder decanol alcohol.

3. The process as defined in claim 1 wherein the resistor is a rutheniumsystem ink, the conductor film is a mixture of seventy percent by weightpure silver powder with at least ten percent by weight of lead borosilicate glass and the remaining percent by weight of the mixture iscomprised of a decanol alcohol in which said powder and glass aresuspended and the substrate is comprised of ninety-six percent purealuminum oxide.

4. The process as defined in claim 1 wherein said first temperature is1000 C. and the lower temperature is substantially 550 C.

5. A thick film resistor for use in an electrically conductive circuit,comprising a selected one of a series of composites made of differentcombinations of positive and negative resistor ink mixes fired inamorphoric form at 1000 C. onto an electrically nonconductive substrateand into a sintered state, the selected composite of the fired resistorand a conductive ink forming a part of said conductive circuit beingheld in overlapping relationship with said resistor on the substrate byfiring said conductor at a temperature that is at its sinteringtemperature and substantially below the temperature level at which saidresistor is changed into its said sintered state to therebysubstantially prohibit diffusion of the conductive material into theresistor and wherein the resulting temperature coefiicient ofresistivity value of the resistor is within a few parts per milliondegrees centigrade from zero.

6. The cermet resistor as defined in claim 5 wherein the selectedcomposite of resistor ink and ink forming said conductive circuit arejointly fired onto the substrate at a temperature that is 550 C.

7. The process as defined in claim 1 wherein the resistor containsdifferent combinations of positive and negative resistor ink mixes, theconductor film is constructed of a cermet conductor mixture which issintered and adhered to the substrate at said lower temperature and thepercent of positive temperature coefficient of resistivity ink requiredin the blend to provide a resistor ink blend having a zero temperaturecoefficient of resistivity is'deterrnined by first plotting therespective positive and negative temperature coefiicient of resistivityvalues of the inks that are to form the blend above and below a zerotemperature coeflicient of resistivity line formed on a semi log graphat separate points located at opposite ends of a characterized log scaleof the graph, reading a line between said plotted points, and readingthe value on said long scale where the intersection of said two linesoccur to obtain the percent of positive temperature coefficient ofresistivity ink in the blend that is required to obtain a zerotemperature coeificient of resistivity mix.

8. The thick film resistor defined in claim wherein the positivetemperature coefficient of resistivity resistor ink is comprised ofsubstantially 20% by weight of RuO 40% by weight Ru and 40% by weight ofglass frit and the negative temperature coefficient of resistivityresistor ink is comprised of substantially 20% by weight of RuO 20% byweight of R and 40% by weight of glass frit.

9. The thick film resistor defined in claim 5 wherein the positivetemperature coeflicient of resistivity resistor ink is comprised ofsubstantially 2% by weight of RuO 40% by weight of R and 40% by weightof glass frit and the negative temperature coefiicient of resistivityresistor ink is comprised of substantially 40% by weight of RuO 20% byWeight of Ru and 40% by weight of glass frit, and wherein the additionof pure gold powder is employed to make the zero temperature coefiicientof resistivity more positive and an oxide of group VIII of the metals isemployed to make the ink more negative.

10. A method for firing a plurality of precision thick film resistorsand associated conductor materials on a common electricallynon-conductive substrate, which resistors may be of the same ordifferent geometric shape, comprising the steps of applying thick filmresistor material to the substrate, drying the thick film resistormaterial and firing said resistor material at its sintering temperaturein amorphous form on the substrate and the subse quent steps of applyingsaid associated conductor on said substrate and in overlappingrelationship to a port-ion of said resistor and firing at a temperaturethat is substantially lower than the first mentioned temperature atwhich said resistor material is fired on said substrate whereby thetendency tor difiusion of said conductor material 16 into said resistoris substantially prohibited and the temperature coefiicient ofresistivity and the sheet resistivity of said resistor is independent ofthe size and geometric shape thereof.

11. A method for firing a plurality of difierent thick film resistorswhich have diiierent geometric shapes and associated conductor materialsonto a common electrically non conductive substrate so that thetemperature coefiicient of resistivity and the resistivity of anyselected one of the resistors can be made independent of its aspectratio, comprising the steps of applying the thick film resistor to thesubstrate, drying the thick film resistor and firing the resistor ontothe substrate at 1000 C. and the sintering temperature of the resistorand the subsequent steps of applying said associated conductor to saidsubstrate and overlapping relationship to a portion of said resistor andfiring the said selected one of said conductors onto the substrate at550 C. which temperature is substantially lower than the first mentionedtemperature at which said resistor is sintered to thereby produce aresistor in which diffusion is substantially prohibted during said lastmentioned firing.

12. A process for firing a thick film resistor and an associatedconductor film onto an electrically non conductive substrate whichprocess inhibits diflusion of material from said conductor film intosaid resistor film whereby the temperature ooefiicient of resistivity ofsaid resistor film is substantially unaffected by said process, saidprocess comprising the steps of first applying the thick reistor film tothe substrate, drying the thick film resistor firing the resistor filmto the substrate at a first temperature that is at the sinteringtemperature of the resistor and thereafter applying the conductor filmto the substrate and in overlapping relationship with said resistor,drying the conductor and firing said conductor film at its sinteringtemperature onto said substrate while said resistor film remainssubstantially below its sintering temperature.

References Cited UNITED STATES PATENTS 3,370,262 2/1968 Morty et al.252-514 X 3,484,284 12/1969 Dates et al. 252-5 13 X 2,920,005 1/ 1960Dearden 2525 19 X 3,248,345 4/ 19 66 Mones et al. 252-514 X 3,553,1091/1971 Hofiman 252514 X 3,583,931 6/l97 1 Bouchard 252-520 3,304,1992/196 7 Faber et a1 252-514 X 2,924,540 2/19'60 DAndrea 25Z5 14 X3,076,908 2/ 1963 Pfaender 25Z5l4 X RALPH S. KENDALL, Primary ExaminerUS. Cl. X.R.

UNITED STATES PATENT oFmcE CERTIFICATE OF CORRECTWN Patent No. 3 788 891Dated January 29 1974 Sergei Schebalin Inventor(s) It is certified thaterror appears in the aboveidentified patent and that said Letters Patentare hereby corrected as shown below:

In the drawing, Figure 1, change "+10" to read +10 Column 1, line 13,cancel "#1"; line 16, cancel "#2"; line 37, "and" should read or line54, insert a comma after "which" and after "example"; line 68,"resistance value" should read resistivity Column 2, line 22, after"therefore" insert a comma line 64, "present" should read prevent Column3, line 10, before "that" insert type line 75 cancel in a non-diffusedman" Column 4,11% L cancel "ner," line 15, after "words" insert a commaColumn 5 lines 3 and 4, cancel "such as"; Column 6, line 42, "TCR",second occurrence, should read TCR' line 67, before the period insert isthen determined line 68, cancel "A" and insert This is accomplished bydrawing a cancel "is drawn". Column 7, lines 12 and 13, "addition blendof the" should read an additional line 33, "other" should read zero TCRline 42, "sheets" should read sheet line 57, "on" should read of line65, after "by" insert the line 66, "bidge" should read bridge Column 8,line 9, after "have" insert a lines 13 and 14, cancel "in Figure 2";line 14, cancel the comma first occurrence, and insert and cancel "onFig. 2"; lines 29 and 32, "10'', each occurrence, to

- l4 line 29, "yet" should read the line 45, cancel "yet"; line 50-,before "value" insert resulting after "14" insert a comma line 52 cancel"to" and insert will be lines 52 and 53, cancel "will be 3% of nominal";line 65, "10" should read ten column 9, line 62, Cancel With" and insertwhich em t ploys a line 63 cancel line 70, cancel "That is" and insertIn other wards line 73 'Exeriments" should read Experiments Column 11,line 15 cancel "at least"; line 16, before "96" insert at least FORMPO-I050 (10-69) USCOMM-DC 60376-P69 U S. GOVERNMENT PRINT NG OFFICEt869.930

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTEGN patent 783 DatedJanuary 29 1974 Sergei Schebalin Page 2 lnventor(s) It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

line 16, cancel "a" and insert at least line 41, after "time" insert forline 49, "aprayed" should read sprayed Column 12, lines 16 and 19, after"conductors" insert 16, 22 Column 13, line 9, after "conductor" insertwhich is after "connected" insert to line 26, after "blending" insert acomma H line 44, "8C" should read 8 line 75, before "time" insert firstColumn 14, line 14, "in" should read and Column 15, line 25,

after "R O insert a comma line 32 "2%" should read 20% Claim 7, line 15,"reading" should read drawing Claim 8, line 6, "20%" should read 40%Claim ll, line 12, after "and" insert 1n Signed and sealed this 24th dayof December 1974.

Attest:

McCOY M. GIBSON JR. C. I IARSHALL DANN Attesting Officer Commissioner ofPatents FORM PC4050 (10-69) USCOMM-DC 60376-P69 U.Sv GOVERNMENT PRINTINGOFFICE 9 93 o 4

